为什么我输入20 20 20和程序提前写好w(20,20,20)结果不一样啊

大神们麻烦跑一下，w(20,20,20)（注释部分）会给出21的结果，如果注释掉自己输入20 20 20，结果就是很大一个数； 蒟蒻看了半个多小时了，这个没法调试啊

#define _CRT_SECURE_NO_WARNINGS 1
//#include <bits/stdc++.h>
#include <stdio.h>
#include <iostream>
using namespace std;
#include <unordered_map>
#include <vector>
#include <math.h> 
#include <list> 
#include <map> 
#include <stack>
#include <queue>
#include <set>
#include <unordered_set>
#include <string>
#include <algorithm>//sort
#include <stdlib.h>
#include <iomanip>
#include <cstring>//memset
#include <limits.h>
/*ios::sync_with_stdio(false);
cin.tie(0);*/

//long long a, b, p;//a^b mod p=s
//long long fp(int c)
//{
//    if (c == 1)return a % p;
//    long long tmp = fp(c / 2);
//    if (c % 2 == 0)
//        return tmp * tmp % p;
//    else
//        return tmp * tmp % p * a % p;
//}

long long a, b, c;
long long arr[25][25][25] = { 0 };
long long w(long long af,long long bf,long long cf)
{
    if (af <= 0 || bf <= 0 || cf <= 0)
        return 1;
    if (af > 20 || bf > 20 || cf > 20)
        return w(20,20,20);
    if (arr[af][bf][cf])
        return arr[af][bf][cf];
    if (af < bf && bf < cf)
    {
        arr[af][bf][cf] =
            w(af, bf, cf - 1) 
            + w(af, bf - 1, cf - 1) 
            - w(af, bf - 1, cf);
        return arr[af][bf][cf];
    }

    arr[af][bf][cf] = w(af - 1, bf, cf) 
        + w(af - 1, bf - 1, c) 
        + w(af - 1, bf, cf - 1) 
        - w(af - 1, bf - 1, cf - 1);
    return arr[af][bf][cf];
}
int main() 
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    //cout << w(20, 20, 20) << endl;
    while (1)
    {
        cin >> a >> b >> c;
        if (a == -1 && b == -1 && c == -1)break;
        cout << "w(" << a << ", " << b << ", " << c << ") = " << w(a, b, c) << endl;
    }
    return 0;
}